Structure and Composition of Cereal Grains

Structure

Cereal grains are the fruit of plants belonging to the grass family (Gramineae). Botanically, cereal grains are a ‘dry’ fruit called a caryopsis (Fig. 1.1). The caryopsis fruit has a thin, dry wall which is fused together with the seed coat.

Kernel structure is important with respect to minimizing damage during grainharvest, drying, handling, storage, milling, and germination and in enhancing nutritional value. There are a few important structural features that the cereal grains have in common. All of the cereal grains are plantseeds and contain three distinct anatomical portions – a large centrally located starch endosperm, which also is rich in protein, protective outer layers such as hull and bran, and an embryo or germ.The seed portion of cereals consists of numerous components which basically include three parts: a seed coat or testa (bran), storage organ or nutritive reserve for the seed (endosperm), and a miniature plant or germ. The fruit tissue consists of a layer of epidermis and several thin inner layers a few cells thick. The aleurone layer which is just below the seed coat, is only a few cells thick, but is rich in oil, minerals, protein and vitamins. Starch and protein are located in the endosperm which represents the bulk of the grain and is sometimes the only part of the cereal consumed. Starch is arranged in the formo ofsub-cellular structures called granules that are embedded in a matrix of protein. The developing endosperm contains protein bodies which become a continuous phase as the grain matures. There is generally a gradient of moreprotein and less starch per cell from the outer to the inner region of the endosperm. The diameter, shape, size distribution and other characteristics of starch granules vary with different cereals. Starch granules range in size from 3-8 µm in rice; 2-30 µm in corn, and 2-55 µm in wheat. Reserve proteins in the endosperm are in the form of smaller ‘protein bodies’ that range in size from 2-6 µm that become disordered and adhere to the starch granules in the mature grain of species like wheat.

Cereals are of plant origin which yield edible grains which are consumed

directly or in modified form as major part of diet and also feed to livestock. Rice and wheat are most important cereals forming part of human food.

The major constituents of the principal cereals are listed in Table 1.1. Cereal grains consist of about two third carbohydrates, mainly in form of digestible sugars and starches. These grains are also an important source of several other nutrients such as protein, calcium, iron, vitamin B complex and dietary fiber. Cereal grains contain 10-14% moisture, 58-72% carbohydrate,8-13% protein, 2-5% fat and 2-11% indigestible fiber. They also provide about 300-350 kcal/100 g of grains. Cereals are deficient in vitaminsA, D, B12 and C.

Cereal Grains

Morphology and nutrient composition

The cereal grains are seeds of the grass family. The word cereal is derived from ‘ceres’, the Roman Goddess of grain. The principal cereal crops are rice, wheat, maize or corn, jowar, ragi and bajra. The term cereal is not limited to these but also flours, meals, breads and alimentary pastes orpasta. Cereal grains provide the world with a majority of its food calories and about half of its protein. These grains are consumed directly or in modified form as major items of diet (flour, starch, oil, bran, sugar syrups and numerous additional ingredients used in the manufacture of other foods) .

The ease with which grains can be produced and stored, together with the relatively low cost and nutritional contribution has resulted in widespread use of cereal foods. They are the staple foods in the diets of most population groups.

The overall structure of all cereal grains is basically similar differing from one cereal to another in detail. The percentage of endosperm, germ, and bran of cereal are 83, 14 ½ and 2 ½, respectively.

Bran or pericarp

The outer layer, epidermis of the cereal consists of thin-walled long rectangular cells. Next to the epidermis is the hypoderm of varying thickness. The innermost layer of pericarp tears during the ripening of the seed and in the mature grain they are represented by a layer of branching hypha-like cells called tube cells. The seed coat or testa is a thin single or double layer. The inner layer of testa of wheat is often deeply pigmented which gives the grain its characteristic colour. Next of tests is a hyaline (nucellar tissue) which is colourless and devoid of any obvious cellular structure.

Aleurone cell layer

The endosperm is surrounded by one or more layers of cells known as aleurone. In wheat, the aleurone is a single layer of thick-walled cubicle cells and constitutes 7 percent of grain weight. The cells contain about 20 percent each of protein, oil, and mineral matter. The cells are also rich in nicotinic acid. The aleurone cells also contain tiny grains of phytic acid with some protein.

Endosperm

The endosperm itself consists of cells of various sizes, shapes, and different composition. The endosperm cell consists mainly of starch and protein, the starch being in the form of spherical granules which are single or tightly packed together and embedded in a matrix of protein. The size and shape of the starch granule in the endosperm cells vary from one cereal to another.

Embryo

The germ or embryo consists of many parts. It is separated from endosperm by scutellum which has the function if mobilizing the stored food in the endosperm and transmitting them to the embryo when the grain germinates. The germ and scutellum are rich in protein and fat. Most of the B vitamins in the grain are present in the scutellum.

Composition and nutritive value

Energy: Cereals are the main source of energy, contributing 70-80 percent of the requirement. Hundred grams give more than 340 kcal of energy.

Carbohydrates: 80 percent of dry matter of cereals is carbohydrate. The two carbohydrates present are crude fibre and soluble carbohydrate. The fibre constituents are cellulose, hemicelluloses and pentosans which are concentrated in all cereals. Small quantities of dextrin and sugars are also present. Free sugars present include simple sugars such as glucose and disaccharides like sucrose and maltose. Of all the cereals, whole wheat, ragi and bajra contain high amount of fibre.

Protein: The protein content of different cereals varies. Rice contains less amount of protein compared to other cereals. The protein content of different varieties of the same cereal also varies. Proteins are found in all the tissues of the cereal grain. Higher concentrations occur in the embryo, scutellum and aleurone layer than in the endosperm, pericarp and testa. Within the endosperm the concentration of protein increases from the centre to the periphery. The types of protein present in cereals are albumins, globulins, prolamines (gliadins) and glutelins. The proportion of these proteins differs in different cereals. The gliadins and glutelins are known as gluten proteins. The gluten has unique elasticity and flow properties which are used for baking bread and other products.Cereals contain 6-12 percent protein, which is generally deficient in lysine.

They provide more than 50 percent of protein requirement as they are consumed in large quantities. Among cereals, rice protein is of better quality than the others. Cereals, when consumed with pulses, the protein quality improves due to mutual supplementation. Cereals are deficient in lysine and rich in methionine. Pulses are deficient in methionine and rich in lysine. Hence, there is improvement in protein quality of both proteins.

Lipids: Lipids are present to the extent of 1-2 percent in wheat and rice, and 3 percent in maize. More lipids are present in germ and bran than in other parts of the grain. Wheat germ contains lipids 6-11 percent and bran 3-5 percent and endosperm 0.8-1.5 percent. Lipid content of maize germ is 35 percent and the bran contains 1 percent. The lipids are mostly the triglycerides of palmitic, oleic and linoleic acid. Cereals also contain phospholipids and lecithin. Considering the amount of cereal consumed it is estimated that fat present in cereals in our diets can meet more than 50 percent of our essential fatty acid requirement. Cereals together with pulses can nearly meet the essential fatty acid requirement of an adult.

Minerals: About 95percent of minerals are the phosphates and sulphates of potassium, magnesium, and calcium. A considerable part of phosphorous in cereals is present in the form of phytin. Phosphorous and calcium present in phytin are not available for absorption. Phytates present in cereals decrease the absorption of iron. Unrefined cereals contain more phytates than refined or polished cereals. Off germination of the grains, the phytate content reduces due to enzymatic breakdown and iron availability is improved.Some mineral elements like copper, zinc, and manganese are also present in very small quantities in cereals.

Cereals are poor sources of calcium and iron particularly rice is a very poor source of these two elements. The content depends upon the extent of polishing. Ragi is a rich source of calcium and iron. Millets (ragi, bajra, jowar) are rich in minerals and fibre. The iron content of wheat is increased during milling where iron rollers are used.

Vitamins: Whole grain cereals are an important source of B vitamins in our diet. Since most of these vitamins are in the outer bran, refining or polishing the grains reduces B vitamin content. Parboiling which includes soaking in water and steaming of paddy results in seeping of vitamins present in outer layer into the grain. Hence, milled and polished parboiled rice retains much of the B vitamins. Maida has less B vitamins than whole wheat flour.Cereals do not contain either vitamin A or C except maize which contains small amount of carotenes. Oils from cereal grains are rich in vitamin E.

Enzymes: Certain grains contain many enzymes and of these the amylases, proteases, lipases and oxidoreductases are of importance. Upon germination α amylase activity increases. The proteases are relatively more in the germ. The lipases of the cereals are responsible for the fatty acids appearing during storage of the cereals and their products.

Glossary

Monocotyledons, also known as monocots, are one of two major groups of flowering plants (or angiosperms) that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots

Poaceae (also known as the Gramineae) is a large and nearly ubiquitous family of monocot flowering plants. Members of this family are commonly called (land) grasses.

Caryopsis is a type of simple dry fruit — one that is monocarpellate(formed from a single carpel) and indehiscent (not opening at maturity) and resembles an achene, except that in a caryopsis the pericarp is fused with the thin seed coat.

The caryopsis is popularly called a grain and is the fruit typical of the family Poaceae (or Gramineae), such as wheat, rice, and corn.

Husk (or hull) in botany is the outer shell or coating of a seed. It often refers to the leafy outer covering of an ear of maize (corn) as it grows on the plant. Literally, a husk or hull includes the protective outer covering of a seed, fruit or vegetable.

Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch,though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet. For example, wheat endosperm is ground into flour for bread (the rest of the grain is included as well in whole wheat flour), while barley endosperm is the main source for Beer production.

Germ of a cereal is the reproductive part that germinates to grow into a plant; it is the embryo of the seed. Along with bran, germ is often a by-product of the milling that produces refined grain products. Cereal grains and their components, such as wheat germ, rice bran, and maize may be used as a source from which vegetable oil is extracted, or used directly as a food ingredient. The germ is retained as an integral part of whole-grain foods

Bran is the hard outer layer of grain and consists of combined aleurone and pericarp. Along with germ, it is an integral part of whole grains, and is often produced as a by- product of milling in the production of refined grains. When bran is removed from grains, the grains lose a portion of their nutritional value. Bran is present in and may be milled from any cereal grain, including rice, corn (maize), wheat, oats, barley and millet. Bran should not be confused with chaff, which is coarser scaly material surrounding the grain, but not forming part of the grain itself. Bran is particularly rich in dietary fiber and essential fatty acids and contains significant quantities of starch, protein, vitamins and dietary minerals.